Why does the Python3 ipaddress.IPv6Network.hosts function omit the first and last IPv6 address?

Question

I noticed that the first and last IPv6 addresses of a given network are omitted:

$ python3
Python 3.3.2 (default, Sep  6 2013, 09:30:10) 
[GCC 4.8.1 20130725 (prerelease)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import ipaddress
>>> print("\n".join([str(x) for x in ipaddress.ip_network("2001:0db8::/120").hosts()]))
2001:db8::1
2001:db8::2
...
2001:db8::fe
>>> 
>>> hex(int(ipaddress.ip_address('2001:db8::fe')))
'0x20010db80000000000000000000000fe'

I believe that for IPv4 this is correct, as those represent the network and broadcast addresses, but I don't believe those exist in IPv6 - at least section-2.5.4 of RFC4291 doesn't seem to mention it. I checked the errata and I don't think the updating RFCs are relevant. The section on interface identifiers also doesn't appear to reserve the first/last addresses.

Does anyone know why these addresses are omitted? Is this a bug in the standard library?

EDIT: Ok, looks like the first address is a Subnet-Router anycast address, so that explains why the first address is omitted.

The last address could be a reserved anycast address, though so is for example 2001:db8::fe/120 (which you can find in the output above), so this is certainly handled inconsistently. Does anybody know why?

Answer 1

This is a bug in the ipaddress module, as unlike IPv4, both the first and last addresses in an IPv6 subnet are valid and usable, though they may have special uses as you've noted.

A quick survey of Python's bug tracker doesn't turn up this bug, so your next step should be to file a bug report on it.